Well if you want Sony gift cards to buy from the Sony Store,
Leaping Gnome has been buying them and sending the codes to members fo quiet a while. Best way would probably be to PM him.
Another option, if you are willing to spend a little time converting would be buy books in Microsoft reader (.lit) format, remove the DRM using ConvertLIT (Google it) and then convert the files using something like Book Designer or Calibar.
That should probably not take more then a minute or two once you figure it out. Search the forum and you'll easily find more detailed instructions.
Ps. and Welcome to the Forum